An amplifier offset voltage may be defined as a voltage that is applied between the two input terminals of the amplifier in order to obtain zero volts at the output. Ideally, the output of the amplifier circuit should be at zero volts when the inputs are grounded. However, in practice, the input terminals are at slightly different voltages due to inherent mismatches of input transistors and components during fabrication of the silicon die. These effects collectively produce a mismatch of the bias currents that flow through the input circuit resulting in a differential voltage at the input terminals of the amplifier circuit. Amplifier offset voltages have been reduced with modern manufacturing processes through increased component matching and improved package materials and assembly but they have not been eliminated and may cause errors in signal amplification during circuit operation.
A chopper circuit is a switching device that converts a fixed DC input signal to a variable DC output signal. Essentially, a chopper circuit is an electronic switch that is used to interrupt one signal under the control of another. Chopper circuits may be used in amplifier circuits to further reduce offset voltages and low frequency noise of an amplifier circuit. One type of chopper circuit is disclosed in U.S. Pat. No. 6,380,801 to McCartney. However, for an offset voltage to be cancelled out while chopping, the offset voltage should remain constant for all chop phases.